Cylinder for internal combustion engine and method of treating inner wall surface of the cylinder

ABSTRACT

There is provided a cylinder for an internal combustion engine, wherein the inner wall surface of the cylinder which constitutes a slide-contacting surface for the piston is treated with a metal plating which is minimal in burden to the environment, excellent in abrasion resistance and hard enough to endure the practical use of the cylinder, thus making it possible to dispense with a finishing work such as honing work. The cylinder is featured in that the inner wall surface of the cylinder which is adapted to be slidably contacted with a piston is electroplated in a plating bath comprising a nickel plating solution containing a boron compound such as trimethylamine borane, dimethylamine borane, etc.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a cylinder made of an aluminum alloy, adapted to be used an internal combustion engine and having a metal plating on the inner wall surface thereof, and also to a method of treating the inner wall surface of the cylinder. In particular, the present invention relates to a cylinder which is suited for use in a small air-cooled two-stroke gasoline engine to be employed in a portable power working machine such as a brush cutter or a chain saw.

[0003] 2. Description of the Related Art

[0004] As for the conventional cylinder for a small air-cooled two-stroke gasoline engine to be employed in a portable power working machine, a typical example thereof is constructed as shown FIG. 2. The cylinder shown in FIG. 2 is generally formed of an aluminum alloy and constituted by an integral body consisting of a barrel portion 2 having a pair of columnar expanded portions 2 a disposed opposite to each other, and a head portion 3 provided with a so-called squish dome-shaped combustion chamber 4. The barrel portion 2 is provided, all over the outer wall thereof, with a large number of cooling fins 19. Further, the head portion 3 is provided with an internal thread 18 for mounting an ignition plug thereon.

[0005] The inner wall surface (cylinder bore face) 9 of the barrel portion 2 is provided with an exhaust port 5 and also with an intake port 6, which are designed to be closed and opened by the movement of the piston 20. The intake port 6 and the exhaust port 5 are disposed opposite to each other in such a manner that they are offset or dislocated level-wise from each other. The columnar expanded portions 2 a are respectively provided therein with a hollow scavenging passageway 7 having an inner wall 7 a of predetermined thickness, each hollow scavenging passageway 7 being spaced away from the intake port 6 and the exhaust port 5 by an angle of 90 degrees in the circumferential direction of the cylinder bore 9. These scavenging passageways 7 are respectively provided, at a downstream end portion (upper end portion) thereof, with a scavenging port 8 which is inclined somewhat upward, i.e. in the direction away from where the intake port 6 of the cylinder bore 9 is located. This pair of scavenging ports 8 is also designed to be opened and closed by the movement of the piston 20.

[0006] The cylinder 1 mentioned above is a so-called binary fluid scavenging type cylinder where a pair of scavenging ports 8 is symmetrically formed with respect to the longitudinal section F-F taken along the middle of the exhaust port 5. However, a so-called quaternary fluid scavenging type cylinder where a pair of scavenging ports is additionally provided therewith (two pairs in total) is also known.

[0007] As for the types of the scavenging passageways, there is also known, in addition to the hollow scavenging passageway provided with an inner wall as shown in FIG. 2 and a scavenging passageway having no inner wall (the side 9 facing the cylinder bore is opened), another kind of scavenging passageway which is provided, at a lower portion thereof, with an opening extending along the length of the scavenging passageway while leaving a half-wall at an upper portion thereof, thereby enabling the air-fuel mixture that has been introduced into the scavenging port from the crank chamber via the scavenging passageway to be contacted with the skirt portion of the piston.

[0008] The cylinder 1 made of an aluminum alloy for use in a two-stroke internal combustion engine is usually manufactured by means of a die casting method such as a high-pressure die casting method, and thereafter, the inner wall surface 9 which is adapted to be slidably contacted with a piston is usually subjected to metal plating treatment (a layer of plating 10).

[0009] Conventionally, the metal plating treatment of the inner wall surface 9 of the cylinder 1 is performed by way of nickel (Ni) plating or hard chromium (Cr) plating. In recent years however, nickel (Ni) plating (electrolytic Ni—SiC plating, electroless Ni—B plating, etc.) is preferentially employed by most automakers. This trend is considered to be attributed to the facts that in contrast to the chromium (Cr) plating where poisonous hexavalent chromium is employed, the toxicity of the plating bath of nickel plating is very low, and that it is possible, through the addition of additives to the plating bath in the nickel plating, to obtain an excellent plating which is comparable to the chromium plating. It is quite conceivable that the employment of chromium plating which is of great burden to the environment will be gradually reduced in the future. Therefore, it is strongly desired to develop a novel plating method for the cylinder in place of the chromium plating. As for the nickel plating in particular, the Vickers hardness of the layer of nickel plating is as low as 350(Hv[50 gf]), thus raising a problem as to how to improve the hardness of the plated layer.

[0010] Additionally, the conventional plating treatment is poor in precision and hence the layer of plating is non-uniform, varying in thickness depending on the regions of the inner wall surface which constitutes a slide-contacting surface for the piston. As a result, it has been considered necessary to perform a finishing work such as honing work after the plating of the inner wall surface.

BRIEF SUMMARY OF THE INVENTION

[0011] The present invention has been made to cope with the aforementioned circumstances, and therefore an object of the present invention is to provide a cylinder for an internal combustion engine, wherein the inner wall surface of the cylinder which constitutes a slide-contacting surface for the piston is treated with a metal plating which is minimal in burden to the environment and hard enough to endure the practical use of the cylinder, thus making it possible to dispense with a finishing work such as honing work.

[0012] Another object of the present invention is to provide a method of treating the inner wall surface of the cylinder, which is capable of obtaining an excellent cylinder as described above.

[0013] With a view to realize the aforementioned objects, there is provided, according to the present invention, a cylinder for an internal combustion engine, which is made of an aluminum alloy and includes an inner wall surface of the cylinder, which is adapted to be slidably contacted with a piston, electroplated in a plating bath comprising a nickel plating solution containing a boron compound such as trimethylamine borane, dimethylamine borane, etc.

[0014] According to a preferable embodiment of the present invention, the layer formed on the inner wall surface by the electroplating is employed, as it is, as a finished surface without being subjected to a finishing work by means of honing, etc.

[0015] Preferably, the layer obtained by said electroplating has a Vickers hardness of 650 to 950(Hv[50 gf]). Further preferably, the deviation in thickness of the layer obtained by the electroplating all over the inner wall surface is confined within 15 μm.

[0016] Further, the cylinder for an internal combustion engine according to the present invention is constructed such that a head portion constituting the combustion chamber of the engine is formed integral with a barrel portion into which the piston is slidably inserted, and is particularly suited for use in a small air-cooled two-stroke gasoline engine to be employed in a portable power working machine.

[0017] In addition, the method of treating the inner wall surface of the cylinder for an internal combustion engine according to the present invention includes an inner wall surface of the cylinder, which is adapted to be slidably contacted with a piston, electroplated in a plating bath comprising a nickel plating solution containing a boron compound such as trimethylamine borane, dimethylamine borane, etc.

[0018] It has been confirmed that when the inner wall surface of the cylinder which is adapted to be slidably contacted with a piston is electroplated in a plating bath comprising a nickel plating solution containing a boron compound such as trimethylamine borane, dimethylamine borane, etc., the hardness of the layer of plating is considerably enhanced. More specifically, while the Vickers hardness of the layer of nickel plating where the aforementioned borane was not added therein was about 350(Hv[50 gf]), the Vickers hardness of the layer of nickel plating where the aforementioned borane was added therein was as high as 700(Hv[50 gf]), i.e. twice as high as that of the layer of nickel plating obtained without the addition of aforementioned borane.

[0019] It has been also confirmed that the deviation in thickness of the layer of nickel plating all over the inner wall surface is confined within 15 μm, and that the layer of nickel plating can be adhered with high precision in conformity with the dimension of cutting work.

[0020] Therefore, it is now possible, according to the present invention, to provide a cylinder for an internal combustion engine, wherein the inner wall surface of the cylinder which constitutes a slide-contacting surface for the piston is treated with a nickel plating which is minimal in burden to the environment, excellent in abrasion resistance and hard enough to endure the practical use of the cylinder, thus making it possible to dispense with a finishing work such as honing work. Further, it is also possible, according to the present invention, to provide a method of treating the inner wall surface of the cylinder, which is capable of obtaining an excellent cylinder as described above.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0021]FIG. 1 is a graph showing the results of the distribution of the thickness of the layer of plating, which were obtained through the measurement of the thickness of the layer of plating in one embodiment of the cylinder for an internal combustion engine according to the present invention; and

[0022]FIG. 2 is a longitudinal sectional view illustrating one embodiment of the cylinder for an internal combustion engine.

DETAILED DESCRIPTION OF THE INVENTION

[0023] One embodiment according to the present invention will be explained in detail with reference to the drawings.

[0024] As in the case of the cylinder shown in FIG. 2, one embodiment of the cylinder for an internal combustion engine according to this embodiment is adapted to be employed in a small air-cooled two-stroke gasoline engine (displacement: about 20-150 mL) to be employed in a portable power working machine such as a brush cutter or a chain saw. However, while the cylinder shown in FIG. 2 is constructed such that the scavenging passageway 7 is formed of a hollow scavenging passageway having an inner wall 7 a, the cylinder according to this embodiment is not provided with the inner wall 7 a (the inner wall surface 9 side of the cylinder is opened). Since the features other than this inner wall surface 9 side are substantially the same as those of FIG. 2, this embodiment will be explained as follows with reference to the cylinder 1 shown in FIG. 2.

[0025] The cylinder according to this embodiment was manufactured by a process wherein a crude cylinder was cast-molded from an aluminum alloy by means of a high-pressure die casting method, and then the inner wall surface 9 thereof was cut-worked into a predetermined dimension so as to enable the piston to move slidably therein, which was followed by an electroplating of the inner wall surface 9 by making use of a conventional nickel plating apparatus employing a nickel plating solution containing therein trimethylamine borane (TMAB), thereby forming a nickel plating layer 10 so as to enhance the abrasion resistance of the inner wall surface 9. In this case, trimethylamine borane was added at a ratio of 0.5 g/L (the nickel plating solution).

[0026] Next, in order to investigate the characteristics and features of the nickel plating layer 10 formed on the inner wall surface 9 of the aluminum alloy cylinder for an internal combustion engine, the inner wall surface 9 being designed to be slidably contacted with the piston, and the nickel plating layer 10 being formed through an electroplating in a plating bath comprising a nickel plating solution containing trimethylamine borane as described above, a comparative test was performed using a sample where a nickel plating was performed without the addition of the TMAB. The conditions for the nickel plating are shown in the following Table 1, and the results obtained are explained below. TABLE 1 Ni plating Ni plating without TMAB with TMAB Plating time (min.) 7 6 Electric current (A/piece) 30 30 Voltage (V) 4.48 4.74 Temp. of bath (° C.) 46 46 Conc. of NiSO₄ (g/L) 280 280 Conc. of NiCL₂ (g/L) 50 50 Conc. of boric acid (g/L) 40 40 pH 4.43 4.43

[0027] (I) The hardness of the nickel plating layer 10 was measured by making use of a micro-Vickers hardness meter MVK-H1 (available from AKASHI Co. Ltd.). As a result, while the Vickers hardness of the layer of nickel plating where the aforementioned TMAB was not added therein was found to be about 350(Hv[50 gf]), the Vickers hardness of the layer of nickel plating where the aforementioned TMAB was added therein according to this embodiment was found to be as high as 700(Hv[50 gf]).

[0028] (II) Further, the thickness of the nickel plating layer 10 was measured so as to investigate the distribution of the thickness, the results being illustrated in the graph of FIG. 1. In this FIG. 1, the ordinate represents the layer thickness (μm), and the abscissa represents the distance (mm) as measured from a clearance groove 12 for working the combustion chamber (see FIG. 2). In this FIG. 1, the thickness of the nickel plating layer 10 all over the inner wall surface 9 is represented by lines, i.e. by the line (A) for the region disposed on the exhaust port 5 side; by the line (B) for the region disposed on the intake port 6 side; and by the line (D) for the region disposed on the scavenging passageway 7 (scavenging port 8) side. Since the regions corresponding to the exhaust port 5, the intake port 6 and the scavenging passageway 7 (opening) were of course not subjected to the metal plating, these regions are made vacant in each of these lines (A), (B) and (D). As seen from this graph, the layer thickness of every region of the plated layer 10 according to this embodiment was confined within the range of 28 to 40 μm, so that the deviation in layer thickness in every region was confined within 15 μm. Furthermore, the nickel plating layer 10 was found as being adhered with high precision in conformity with the dimension of cutting work. Therefore, it was possible, according to the cylinder of this embodiment, to dispense with a finishing work such as honing work.

[0029] (III) When the surface conditions of the plated layer were observed by making use of a light microscope, the grain boundary of the plated layer appeared unclear and the surface of the plated layer appeared roughened in the case of the nickel plating layer where the TMAB was not added thereto. Whereas, in the case of the plated layer 10 according to this embodiment where the TMAB was added thereto, the grain boundary of the plated layer was quite clear, and the surface condition of the nickel plating layer was quite similar to that of the conventional hard chromium plating or that of iron plating.

[0030] (IV) When the cross-sectional surface of the plated layer was observed by making use of a scanning electron microscope, the grain of plating was recognized as having grown in the vertical direction from only the raw material region in the case of the nickel plating layer where the TMAB was not added thereto, and also the new generation of grain of plating was not recognized inside the plated layer. Whereas, in the case of the plated layer 10 according to this embodiment where the TMAB was added thereto, the grain of plating appeared like a fine scale-like pattern, and the new generation of the grain of plating also appeared in the plated layer thus grown.

[0031] While in the foregoing, one embodiment of this invention has been explained in detail for the purpose of illustration, it will be understood that the construction of the device can be varied without departing from the spirit and scope of the invention.

[0032] For example, while trimethylamine borane was employed in the aforementioned embodiment, it is possible to employ other kinds of boron compounds such as dimethylamine borane.

[0033] As apparent from the above explanation, according to the present invention, it is possible to provide a cylinder for an internal combustion engine, wherein the inner wall surface of the cylinder which constitutes a slide-contacting surface for the piston is treated with a metal plating which is minimal in burden to the environment, excellent in abrasion resistance and hard enough to endure the practical use of the cylinder, thus making it possible to dispense with a finishing work such as honing work. It is also possible, according to the present invention, to provide a method of treating the inner wall surface of the cylinder, which is capable of obtaining an excellent cylinder as described above. 

1. A cylinder for an internal combustion engine, which is made of an aluminum alloy and is featured in that an inner wall surface of said cylinder which is adapted to be slidably contacted with a piston is electroplated in a plating bath comprising a nickel plating solution containing a boron compound such as trimethylamine borane, dimethylamine borane, etc.
 2. The cylinder according to claim 1, wherein a layer formed on said inner wall surface by said electroplating is employed, as it is, as a finished surface without a finishing work by means of honing, etc.
 3. The cylinder according to claim 1 or 2, wherein the layer formed by said electroplating has a Vickers hardness of 650 to 750(Hv[50 gf]).
 4. The cylinder according to any one of claims 1 to 3, wherein a deviation in thickness of the layer formed by said electroplating all over said inner wall surface is confined within 15 μm.
 5. The cylinder according to any one of claims 1 to 4, which is adapted to be used as a cylinder for a small air-cooled two-stroke gasoline engine to be employed in a portable power working machine, said cylinder being constructed such that a head portion constituting the combustion chamber of the engine is formed integral with a barrel portion into which the piston is slidably inserted.
 6. A method of treating the inner wall surface of the cylinder for an internal combustion engine, said method comprising a step of electroplating an inner wall surface of said cylinder which is adapted to be slidably contacted with a piston by making use of a plating bath comprising a nickel plating solution containing a boron compound such as trimethylamine borane, dimethylamine borane, etc. 